The overall goal of the proposed research is to develop non-invasive optical technologies for the real-time, quantitative measurement of optical and physiological properties in small tissue volumes. Currently, optical techniques that employ the detection of diffusely transmitted or reflected light are most often used in conjunction with diffusion-based optical transport models to either (a) image centimeter-thick, highly- scattering, heterogeneous tissues with approximately 5mm. spatial resolution or (b) quantify optical and physiological properties of large, highly-scattering, homogeneous tissue volumes (>50 mm[3]). Although such techniques can also be used to measure a volume-averaged impact of localized heterogeneous structures, the current inability to accurately characterize light transport on small length scales (equal to or < 5mm) significantly hampers the possibility of accurately quantifying optical properties in small, well-defined tissue volumes. Here, we propose a comprehensive theoretical, computational, and experimental approach to substantially extend diffusion approximation limits by enabling the assignment and quantification of optical and physiological properties to small localized tissue volumes (approximately 2-50 mm3) of arbitrary albedo. These properties have been shown to be sensitive, quantitative measures of cellular and extracellular morphology and biochemical composition. Such a capability will spur the development of novel, compact optical probes with broad application including early detection of dysplastic transformation of epithelial tissue structure and composition, intraoperative/endoscopic surgical guidance, as well as diagnostic feedback for real-time monitoring and control of photodynamic, hyperthermal, cryogenic, and coagulative therapies. Such probes are also valuable for basic biological studies in artificial tissue and pre-clinical animal models where the spatial scales probed are inherently small. The proposed research aims to fully develop both a novel optical modeling approach to describe light transport on sub-millimeter length scales as well as computational algorithms to determine optical properties from photon migration measurements made in small tissue volumes. These methods will be extensively tested and validated through the experimental measurement and computational processing of light signals that result from propagation through realistic tissue phantoms. We will apply these newly developed photon migration methods, in conjunction with thick-tissue microscopy techniques, to examine artificially- engineered tissue models for normal and dysplastic epithelia. This latter study will allow us to investigate the interrelationships between microscopic tissue morphology and composition and mesoscopic optical absorption, scattering and anisotropy coefficients provided by photon migration methods.